First tell the dog to sit then:
Some dogs will do it naturally just hold a treat around them
Or say "SHAKE!" and bring the dogs paw up as you say it then give them a treat
try over and over
You hold a cookie up while they're sitting. Grab they're paw, shake it, and say "Good girl (or boy)!" Give them the cookie. Do that about once a day and start making them do it themselves. That's...